Excessive or prolonged exposure to hydrogen sulfide, a highly toxic gas produced naturally by decaying organic matter. Hydrogen sulfide is sometimes called “sewer gas” or “stink damp.” It is a colorless, flammable, very toxic gas with a characteristic rotten egg odor. Hydrogen sulfide poisoning chiefly occurs through inhalation of the gas, although it can occur through direct contact with the liquefied gas used in certain industrial processes. The gas is naturally produced and released from sewage sludge, liquid manure, sulfur hot springs, and natural gas.